Trailer Lines

[OakRidgeMack]

I have a R-400 straight truck and i am having some problems figuring out how you would connect lines to the truck line so that the towed trailer brakes worked and are also functional?

[Freightrain]

I can't give you alot of detail, but basically you'll need a tractor protection valve installed to feed a trailer. Seems there are air line schematics onlines somewhere. Might be the Bendix website?

[OakRidgeMack]

Yea i know that there is alot of places that you can look but im just collecting thoughts on here....thanks freight....all helps to me

[Freightrain]

I originally pulled my very old, broken trailer lines off as I didn't plan on pulling an "air" type trailer. I've since put a new TP valve and lines out back to make it possible again. It's a pretty simple hook up really other then mounting your new dash control.

This is where John Evans(truknut) used to jump in and give all the details. He's been AWOL for some time now.

[ThaddeusW]

OakRidgeMack,

I assume you want to add trailer air lines correct?

Well if you only have a yellow park knob then your going to need a few parts to get yourself setup. Just like Larry said, a tractor protection valve and the red trailer supply dash valve along with a trolley valve (trailer hand brake) and a few other valves.

Ok if you truck is older than 1980 then you most likely have the single circuit air brake system. If its a single circuit system then pluming will be a bit easier. Get a line from the brake supply tank and plumb it to the dash valve supply port and the trolley valve supply port. Then plumb the dash valve delivery port to the emergency/supply port on the tractor protection valve. next, using a three way check valve tee off your foot valves delivery port and the trolley valve delivery port and connect that line to the service line supply port on the tractor protection valve. Then just run the red emergency/supply line and blue service line to a set of glad hands and your set.

Few notes:

The trailer supply knob has a third control port that needs air pressure to allow the charging of the trailer air tanks. You plumb that port to a tee you install after the yellow park dash valve. This enables the yellow knob to not only park the truck but also the trailer as well.

A three way check valve allows two different air sources to supply a single delivery port.

Any more question and I can whip up a diagram for you.

[OakRidgeMack]

ThaddeusW,

Yes i do want to add trailer air lines. The truck is a 1972 and there already is a hand brake on the column that is hooked up to the rear service brakes of the truck.

That diagram would help me alot...and rubber air hoses will work instead of the original metal type, right?

[Freightrain]

I'd use plastic for adding the controls. Much simplier to hook up and you don't have to worry about degrading rubber later on.

[ThaddeusW]

Ok diagram time!

This is a very basic single circuit air brake diagram for a 2 axle strait truck. The single circuit was replaced by a dual circuit system back around 1978-1980.

A) Air compressor

B ) Governor

C) Air dryer (may or may not be present on your truck or plumbed the same)

D) Wet tank. This is the primary air storage tank that supplies accessories (air wipers, suspension, seat, horn etc.) and the brake system.

E) Brake tank. stors air for the brake system. proteced from the wet tank by a check valve so a badly leaking accessory line wont drain the brake tank.

F) Pressure gauge in the cab to monitor system pressure.

G) Foot valve, single circuit type.

H) quick release valves are usually found mounted right to the axle and exhaust the air pressure close to the chambers to improve brake response.

I) front brake chambers

J) Parking dash valve

K) relay valve. This allows the rear brakes to respond quicker to the foot valve. The controll port just meters air pressure from the brake tank to the rear brake chambers.

L) Rear brake chambers. Blue is the normal service chamber and red is the spring emergency/parking chamber.

M) Inversion valve or anti compunding valve. Prevents the release of the spring chambers when your foot is on the brake pedal to avoid damage to the brake chambers.

Ok now to add a trailer you need to make the following connections:

A) Trailer air supply dash valve.

B ) Trolly Valve

C) Three way check valve. Prevents the trolly valve from applying the tractors brakes while allowing the tractor to controll both.

D) Tractor protection valve. A simple valve that uses the emergency supply pressure to hold the service line open. In the event of a leak or break away the tractos brakes are isolated to prevent loss of brakes.

E) Service line and glad hand for trailer

F) Emergency/supply line and glad hand for trailer

Now this is a very simple setup and your trucks system can vary but this is a good guideline. Find an open port on your brake tank, tap a line off the foot valve delivery port and a line off the delivery port off the parking dash valve and you are set. Then some simple plumbing and you have trailer air lines!

Like freightrain said use the plastic DOT rated tubing. You can get valves with quick connect ports that the tubing just slides and locks into. No hose barbs or clamps to futz with. You can buy it online from Mcmaster Carr and they also have tube to pipe adaptors so you can hook into your existing system. Just type DOT into the search.

Any further questions, let us know.

[CML Service]

Just a quick note. Your air system for a 1972 truck would be known as pre FMVSS-121. Those systems generally didn't use relay valves. Speaking from personal expirience without relay valves to help time the truck and trailer brakes you may find the a newer trailer tends to lock the wheels before the truck especially when empty. On my B model I converted the entire system to post FMVSS-121 standards. Now even a panic stop with empty dump trailer or tag along everything stops straight. It was well worth the time and effort for my situation. I can be confident that my truck will stop reliably when empty or grossing 80,000.

If this will be a truck that will pull a loaded trailer take the time and do your research. Relay valves have a rating for crack pressure (when they start to work). This will help you engineer a truck and trailer system that will work correctly together.

Chuck

[ThaddeusW]

Just a quick note. Your air system for a 1972 truck would be known as pre FMVSS-121. Those systems generally didn't use relay valves. Speaking from personal expirience without relay valves to help time the truck and trailer brakes you may find the a newer trailer tends to lock the wheels before the truck especially when empty. On my B model I converted the entire system to post FMVSS-121 standards. Now even a panic stop with empty dump trailer or tag along everything stops straight. It was well worth the time and effort for my situation. I can be confident that my truck will stop reliably when empty or grossing 80,000.

If this will be a truck that will pull a loaded trailer take the time and do your research. Relay valves have a rating for crack pressure (when they start to work). This will help you engineer a truck and trailer system that will work correctly together.

Chuck

Good point Chuck. I haven't seen any pre-fmvss 121 brake diagrams save for one from a Canadian commercial driving manual.

But all in all the simple trailer plumbing diagram remains unaffected. But your experience with trailer wheels locking up definitely shows the advantages of the newer system with relay valves.